1. Field of the Invention
The present invention relates to a photolithography process, and more particularly to a photolithography process with projection exposure.
2. Description of the Prior Art
Photolithography is a well-known process for transferring geometric shapes present on a mask onto the surface of a silicon wafer. In the field of IC lithographic processing, a photosensitive polymer film called photoresist is normally applied to a silicon substrate wafer and then allowed to dry. An exposure tool is utilized to expose the wafer with the proper geometrical patterns through a mask by means of a source of light or radiation. After exposure, the wafer is treated to develop the mask images transferred to the photosensitive material. These masking patterns are then used to create the device features of the circuit.
A simplified diagram of a conventional exposure tool is shown in FIG. 1. As can be seen, light source 100 projects light waves 108 through opening 102 in aperture stop 101. Opening 102 is commonly referred to as the pupil of the aperture stop. Condenser lens 105 collects the light from the opening 102 and focuses it on mask 106 such that the mask 106 is evenly illuminated. When illuminating beam 103 passes through mask 106, imaging beam 109 is generated. Imaging beam 109 is projected through projection lens 107 such that the image of the pattern on the mask 106 is focused onto the silicon wafer 110. As can be seen in FIG. 1, the opening 102 is situated in the center of aperture stop 101. Because of this, illuminating beam 103 is projected along the optical axis (dashed line 104) from the opening 102 to condenser lens 105 and mask 106. This type of illumination method is called “On-axis illumination,”—the name implying that the illumination beam is “on” the optical axis.
One important limiting characteristic of any exposure tool is its resolution limit. The resolution limit for an exposure tool is defined as the minimum feature that the exposure tool can repeatedly expose onto the wafer, which is close to the smallest dimension (referred to as the critical dimension or CD) for many current IC layer designs.
Another important characteristic of an exposure tool is its depth of focus (DOF). The DOF of an exposure tool is defined as the range in which the aerial image (of a near resolution sized feature) will stay in focus. In a lithographic process in which an image is transferred into a photoresist layer, a minimum DOF is required. This minimum DOF ensures that the image remains sufficiently in focus throughout the photoresist layer. Thus, the minimum DOF range is typically greater than or equal to the thickness of the photoresist layer.
The resolution (R) and the DOF of an exposure tool are proportional to the exposure wavelength (λ) and are inversely proportional to the numerical aperture (NAlens) of a projection optical system of the exposure tool, as shown in the following equations (1) and (2):                     R        =                              K            1                    ⁢                      λ                          NA              lens                                                          (        1        )                                DOF        =                              K            2                    ⁢                      λ                                          (                                  NA                  lens                                )                            2                                                          (        2        )            
The DOF of an exposure tool determines the “usable resolution” setting of the exposure tool. For instance, if an exposure tool is able to resolve 0.4 μm features but has a DOF range less than the range needed to clearly focus this feature throughout the photoresist layer, then the 0.4 μm resolution setting can not be utilized. As can be seen, if the range of DOF of an exposure tool can be extended, the usable resolution limit may be decreased and smaller images may be printed.
Referring to FIG. 1, for the conventional photolithography process, the wafer 110 having a photoresist layer formed thereon is exposed with one fixed illumination setting involving numerical aperture (NAlens), sigma value (σ) and exposure energy to generate enough photo-acid for target critical dimensions (CDs).
Sigma value (σ) is a coherence factor, a ratio of the numerical aperture (NAill) of the illuminating optical system of the exposure tool to the numerical aperture (NAlens) of the projection optical system of the exposure tool, which is represented by the equation (3):                     σ        =                              NA            ill                                NA            lens                                              (        3        )            
Referring to FIG. 2A to FIG. 2C, curves A1, A2 and A3 respectively represent the DOF of the dense pattern under illuminating settings from low sigma value (0.35) to high sigma value (0.85) at a constant numerical aperture (NA) 0.68 of the projection optical system. Curves B1, B2 and B3 respectively represent the DOF of the isolated pattern under illuminating settings from low sigma value (0.35) to high sigma value (0.85) at a constant numerical aperture (NA) 0.68 of the projection optical system. It is apparent that the DOF of a dense pattern is extended as sigma value (σ) is increased. On the contrary, the DOF of an isolated pattern becomes shallower.
During the conventional photolithography process with single illuminating setting, the dense pattern and isolated pattern are simultaneously transferred to the photoresist layer from the same mask. However, the results in FIG. 2A to FIG. 2C show that there is not an illuminating setting that is optimum for both the dense and isolated pattern transfer performance. Besides, for the same mask, the critical dimension (CD) is enlarged by light diffraction when the pitch is small, and the critical dimension (CD) is reduced by optical effect when the pitch is increased. Herein, a pitch of a mask is a distance composed of a line width and a space. The mask error factor (MEF), the ratio of a deviation of the critical dimension of a wafer (ΔCDwafer) to a deviation of the critical dimension of a mask (ΔCDmask), severely impacts on the CD control through all pitches. Therefore, a compromise between the dense and isolated pattern transfer performance should be taken.
A conventional photolithography process with double exposures using two masks is employed to overcome the above problems. However, this process creates another issue regarding the alignment accuracy of the two masks. The throughput of the photolithography process is also decreased.
Since the conventional photolithography process with its single exposure method using single illuminating setting cannot fulfill the optimum illuminating setting for patterns at all pitches, an improved photolithography process using multiple exposures with matching illuminating settings is needed.